Elevators are provided with safety systems which will initiate an emergency stop of the car in case of overspeed. The device is generally referred to as a governor, and typically includes a rope which is connected to the car and passes over a pulley in the elevator machine room. The governor rope thus, under normal conditions moves up and down with the car and over the pulley. The pulley is connected to a spinning centrifugally operated trip device which is actuated by car overspeed. When car overspeed occurs, the trip device causes movement of the governor rope to stop which in turn pulls a brake actuator on the car thus stopping the car.
Some forms of elevators are not amenable to the above-described governor system because they do not have a conventional machine room. Such elevators include hydraulic elevators; roped and non-roped, linear induction motor elevators, and elevators which travel on a curved path, such as for the Eiffel Tower in Paris. These elevators, nevertheless, should be equipped with safety governors which will stop the car in the event of overspeed. One solution to providing such elevators with safety governors is to place the governor on top of the car assembly. The governor will include a centrifugally actuated brake tripper, and a rotating member which engages a fixed component of the elevator system in the hoistway.
U.S. Pat. No. 259,951 granted June 20, 1882 to F.W. Voerde discloses a safety attachment for elevators which utilizes a cable stretched from the top to the bottom of the elevator hoistway and wrapped around a pulley mounted on the elevator platform. The rotational speed of the pulley is proportional to the speed of the elevator in the hoistway. Several deficiencies are found in the system shown and described in this patent. Firstly, the pulley cable drags over the elevator platform, and is disposed in a very wide groove in the pulley. These two factors will result in excessive rope wear caused by abrasion of the rope by the platform, and by the rope sliding back and forth in the pulley groove. Excessive noise will also be created. These conditions will be further exacerbated by the fact that the rope in the Voerde system is drawn off and apparently fastened to the side of the hoistway. Another difficulty with the disclosed system which is caused by tying the ends of the rope in place in the hoistway arises from rope stretch which will occur from changes in humidity in the environment in the hoistway. Proper rope tension on the pulley thus cannot be maintained, and, as a result, accuracy and dependability of the system cannot be assured.
U.S. Pat. No. 4,662,481 granted May 4, 1987 to K.E. Morris, et al., disclosed an elevator system which includes a safety device mounted on the elevator car. The safety assembly includes a roller which rides on one of the elevator guide rails. The roller will be free wheeling so long as the speed of the elevator remains below a predetermined safe velocity. If an excessive elevator speed is experienced, the excessive roller speed will trip a centrifugal brake which will lock the roller against rotation. The locked roller is then dragged up the guide rail pulling a cable which trips a safety brake on the car. A problem which resides in this approach relates to the ability to develop enough frictional force between the roller and rail to be able to trip the elevator safety. Rail deflections due to typical rail bracket spacing and car ride considerations may not allow sufficient post-post forces to be developed to assure tripping of the elevator safety with this arrangement.